I have found on our 17,000 mile summer trip that our GMC mileage is
sensitive to cursing speed, head winds and hills. We averaged 9.7 mpg over
the entire trip using 85 to 87 octane gas. We normally drive the speed
limits + 5mph on the freeways. That included such highs as 12.7 mpg when we
were cursing on the back roads of new England at 25 with occasional bust of
55 mph. The low of 7.4 was when we were cursing across Oklahoma flat lands
at 80 mph against 25 mph head winds. The engine and carburetor was rebuilt
and tuned on a dyno along with the ignition system. The carburetor was
adjusted to have the four barrels come in at 5 rather than 9 inches of
vacuum. On hills, the four barrels come in to maintain cursing speed. The
best gas cost saver that I made was to install the Platinum Gassaver which
enable the 455 to use 85 octane without pinging. It cost $168 but saved over
$350 in using lower octane - and it is good for 30,000 miles so I have some
more savings to go using 85/87 octane [1-800-less-gas]. Most of the gas in
the west in the summer has ethanol, so expect a 7% loss in mileage due to
that. I did not notice a major difference due to altitude except the ethanol
boil over problem at high altitudes. We have a 23' that weighs in at 10,100
with full tanks and all our stuff for traveling, including us. We do not
tow.
Compared to other class A gasoline engine motor owners we have talked to,
our GMC is a fuel miser. The new Ford V-10 with fuel injection is getting
better than 10 mpg in smaller class C rigs. If gas mileage is really
important, drive slower and only use two barrels.
I remember back in the fuel rationing days when a friend traded his 72 Olds
Toronto in for a Honda Civic because he never got better then 10 mpg [70 mph
speed limits] on the Olds. He lived far enough away that he could not buy
enough gas to get back and forth to work. 9 to 10 mpg on a full sized car in
the early 70's was normal. In the late 60's all the cars had big engines
with high compressions and got much better mileage. I had a "69 with a 429
with 11:1 compression ration and a four barrel carburetor that got 18 to 20
mpg at 70 mph on 103 octane. Had to really retard it to run on 93 octane and
it got terrible mileage when detuned, I had to stuff my foot into it to get
it to move at all. It was a different car when detuned to run on the new top
octane available
sensitive to cursing speed, head winds and hills. We averaged 9.7 mpg over
the entire trip using 85 to 87 octane gas. We normally drive the speed
limits + 5mph on the freeways. That included such highs as 12.7 mpg when we
were cursing on the back roads of new England at 25 with occasional bust of
55 mph. The low of 7.4 was when we were cursing across Oklahoma flat lands
at 80 mph against 25 mph head winds. The engine and carburetor was rebuilt
and tuned on a dyno along with the ignition system. The carburetor was
adjusted to have the four barrels come in at 5 rather than 9 inches of
vacuum. On hills, the four barrels come in to maintain cursing speed. The
best gas cost saver that I made was to install the Platinum Gassaver which
enable the 455 to use 85 octane without pinging. It cost $168 but saved over
$350 in using lower octane - and it is good for 30,000 miles so I have some
more savings to go using 85/87 octane [1-800-less-gas]. Most of the gas in
the west in the summer has ethanol, so expect a 7% loss in mileage due to
that. I did not notice a major difference due to altitude except the ethanol
boil over problem at high altitudes. We have a 23' that weighs in at 10,100
with full tanks and all our stuff for traveling, including us. We do not
tow.
Compared to other class A gasoline engine motor owners we have talked to,
our GMC is a fuel miser. The new Ford V-10 with fuel injection is getting
better than 10 mpg in smaller class C rigs. If gas mileage is really
important, drive slower and only use two barrels.
I remember back in the fuel rationing days when a friend traded his 72 Olds
Toronto in for a Honda Civic because he never got better then 10 mpg [70 mph
speed limits] on the Olds. He lived far enough away that he could not buy
enough gas to get back and forth to work. 9 to 10 mpg on a full sized car in
the early 70's was normal. In the late 60's all the cars had big engines
with high compressions and got much better mileage. I had a "69 with a 429
with 11:1 compression ration and a four barrel carburetor that got 18 to 20
mpg at 70 mph on 103 octane. Had to really retard it to run on 93 octane and
it got terrible mileage when detuned, I had to stuff my foot into it to get
it to move at all. It was a different car when detuned to run on the new top
octane available
